1. Field of the Invention
This invention relates to a magneto-optical disk having a grooved substrate.
2. Prior Art
Most magneto-optical disks are of the structure having a magneto-optical recording layer on a grooved substrate. Signals are recorded in and reproduced from the recording layer within the groove. During recording and reproducing operation, tracking control and track counting for search are carried out by detecting the intensity of reflected light from near the groove.
Parameters associated with the control include a push-pull signal level and radial contrast during tracking in the grooved region.
The push-pull signals are tracking signals obtained in tracking control by a push-pull method. The push-pull method uses a two-divided photodetector consisting of two light-receiving sections each in the form of a photo diode disposed symmetrically with respect to the track center. The two light-receiving sections receive the light reflected and diffracted by a groove or pit on the disk, and a tracking error is detected from a differential output of the photo diodes. The push-pull signal level P-P is represented by EQU P--P=(I.sub.1 -I.sub.2)/(I.sub.1 +I.sub.2)
wherein I.sub.1 and I.sub.2 are outputs of the two light-receiving sections. Too low push-pull signal levels can cause a failure of normal tracking whereas too high push-pull signal levels become unbalanced with other optical properties and would introduce noise-in focus servo signals depending on the type of optical head.
The radial contrast RC in the grooved region is represented by EQU RC=2.vertline.I.sub.L -I.sub.G .vertline./(I.sub.L +I.sub.G)
wherein I.sub.L and I.sub.G are outputs of a signal from the land and the groove, respectively, when a low-pass filter is used. From RC outputs, the number of tracks that the optical head has jumped over and the moving direction or polarity of the optical head are known. Too low radial contrast values can introduce errors in track counting and polarity judgment whereas too high radial contrast values would render the servo system unstable due to disturbance noise.
Both the push-pull signal level and radial contrast vary with the width and depth of the groove and C/N also varies therewith. More particularly, the radial contrast increases as the groove depth increases (in the case of a groove depth.ltoreq..lambda./4n wherein .lambda. is a light wavelength and n is a refractive index) and as the groove width increases. The push-pull signal level decreases as the groove depth decreases (in the case of a groove depth .ltoreq..lambda./8n) and as the groove width increases (in the case of a groove pitch of 1.6 .mu.m and a groove width .ltoreq.0.8 .mu.m). The C/N ratio increases as the groove width increases. It is then difficult to have a satisfactory push-pull signal level and radial contrast value as well as an acceptable C/N ratio.